It is well known to treat patients with medicaments contained in an aerosol, for example, in bronchodilator therapy. It is also known to use for such therapy, medicaments which are contained in an aerosol and are administered to a patient by means of an inhalation device comprising a tubular housing or sleeve in which the aerosol container is located and an outlet tube leading out of the tubular housing. The aerosol containers used in such inhalation devices are designed to deliver a predetermined dose of medicament upon each actuation by means of an outlet valve member at one end which can be opened either by depressing the valve member while the container is held stationary or by depressing the container while the valve member is held stationary. In the use of such devices, the aerosol container is placed in the tubular housing with the outlet valve member of the container communicating via a support with the outlet tube, for example a nozzle or mouthpiece. When used for dispensing medicaments, for example in bronchodilation therapy, the housing is then held by the patient in a more or less upright condition and the mouthpiece or nozzle of the inhalation device is placed in the mouth or nose of the patient. The aerosol container is pressed towards the support to dispense a dose of medicament form the container which is then inhaled by the patient.
A problem arising from use of such known devices is that the patient cannot determine the amount of medicament in the container at any given time. In an extreme case this could mean that the patient, possibly suffering from severe bronchospasm and needing a dose of medicament, will find that the container will not dispense a dose because its contents have already been exhausted.
In solution to the above described problem there has been suggested the use of dose indicating devices, which typically count the number of doses delivered from or remaining in the aerosol container, thereby enabling the patient to determine how much medicament is available in the container for future use. Typically, the dose indicating device has an indexing (i.e. counting) mechanism actuated by movement of the container relative to the housing, wherein a preset amount of relative movement results in a count being indexed.
U.S. Pat. No. 4,817,822 describes an aerosol dispenser having a dose indicating device which, in a first embodiment is removably attached to the end of the protruding portion of the aerosol container. The operating mechanism of the dose counter is located within a housing which extends from the end of the aerosol container along the external surface of the tubular housing.
U.S. Pat. No. 4,817,822 describes a dispenser having a dose indicating device in which the operating mechanism of the dose indicating device is located within a compartment in the housing and is actuated by means of an actuator member attached to the aerosol container.
WO96/16686 describes an aerosol dispenser wherein the operating mechanism of the dose indicating device is electronic and wherein the actuating member comprises a microswitch set into the wall of the housing. The electronic counting mechanism and microswitch are contained within a hermetically sealed enclosure.
U.S. Pat. No. 5,482,030 describes an aerosol dispenser having a mechanical dose indicator device located in and connected to the housing in the vicinity of the outlet tube of the aerosol container when fitted.
Many different pharmaceutical products are sold in the form of aerosol containers requiring different sized container bodies and/or valves according to the required specifications. It is therefore normal for there to be dimensional variations between different aerosol containers. Even between the same products there can be dimensional variations due to manufacturing tolerances.
A problem which is common to all of the dose indicating devices discussed above is that the indexing mechanism, which is actuated by relative movement between the container body and housing, lacks any means of compensating for different pre-actuation positionings of the indexing mechanism relative to the container arising from, for example, dimensional variations between different aerosol containers. Hence, the indexing mechanisms must be dimensioned according to the product with which they are to be used, and so will not be interchangeable with other products. Furthermore, in order for the indexing mechanism to record a count accurately the dimensions of the components of any particular device must be manufactured to the required high tolerances.